In a winding machine such as a cross bobbin winding machine, thread is deposited in a spiral configuration during winding onto a spool. A thread guide reciprocates back and forth along the length of the spool in an alternating left and right hand spiral with the pitch of the spiral reversing at the spool end. Because of this, the winding speed of the thread is not equal to the peripheral speed of the rotating spool but equals the peripheral speed divided by the co-sine of the pitch angle.
The reversal of the pitch direction does not occur in zero time but in a finite time dependent on the type of thread guide being used. The thread is not deposited at the spool ends in a sharp reversing angle, but in a reversing arc determined by the mechanics of the thread guide elements. Consequently, the pitch angle changes to zero in a finite time and then increases again in the opposite direction to its maximum value. Because of this, the thread speed during the back and forth movement of the thread constantly changes. Thus, when the spool rotates at a constant peripheral speed, the thread speed is not constant.
A further cause of non-constant thread speed is the influence of the so-called thread triangle. The thread triangle is formed by both the reversing points of the thread guide and an upper fixed point which generally lies above the center of the traverse path of the thread guide along the base of the thread triangle. The upper fixed point is spaced from the traverse path of the thread guide by a distance which can be considerable and is generally from two to four times the length of the traverse stroke path. When the thread guide moves from the middle of the traverse path to one distal end thereof, the distance between the thread guide and the fixed point increases. With a constantly rotating spool taking up the thread, this alteration or change in distance causes an increase in the thread speed. When the thread moves in the opposite direction toward the middle of the traverse path, the thread speed decreases because of the decreasing distance between the thread guide and the fixed point. Thus, with a constantly rotating spool, the thread speed is not constant.
Both of these speed alterations are unavoidable and have undesirable effects on the direct winding of threads which are delivered at a takeup zone at constant speed from a processing machine. A medial thread winding speed and thread winding tension may be selected by adjusting the ratio of the winding speed to the delivery speed in a direct winding process. However, because of the constantly changing speed of thread during the winding procedure, irregular thread tension occurs. The resultant differences in thread tension can reach values which can drop from the medial tension to zero and the tension peaks can come to two to five times the value of the medial tension.
It is known to equalize the constant alteration in thread tension by passing the thread over a guide pulley as it runs from the fixed point to the traversing thread guide. The guide pulley is mounted on a pivoting arm and the wrapping angle of the thread is approximately 180.degree.. Such a pivoting arm is also called a jumper arm. The pivoting arm is intended to regulate the thread tension to a constant value as a result of its continual pivoting by means of a counter force. Because the pivoting arm is an inertial element, the regulating facility is difficult and sluggish.
Attempts have been made to replace the guide pulley by having the thread blown in a reverse manner by a stream of fluid medium. In this attempt, the blower device is located at the end of the arm and forms the return momentum for the deflection of the arm. It is necessary to guide the thread around further turning units such as pins or the like on both sides of the blower device so that varying sizes of loops may be formed during the blowing operation. However, even in this case, the device is not free of inertia. Moreover, such a device is bulky because additional turning of the thread in the thread run must be incorporated. In addition, the device is wasteful of energy. That is, a considerable volume of air is consumed because the thread loop must be maintained by a blown jet of fluid medium and a continuous air cushion must be maintained at a relatively high pressure.